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Infiltration methods are designed to help restore the natural hydrologic system and promote water quality by allowing stormwater to percolate into the ground. Infiltration of stormwater reduces the amount of rainfall that becomes runoff. Infiltration also improves groundwater recharge.

The success of an infiltration/recharge system depends on the type of soils and

groundwater system in the area. Soils with low permeability and shallow bedrock depths are poor locations for infiltration methods. Sediments, oils, and other debris can cause clogging of the soil surface. Adequate inspection and maintenance must be provided. Infiltration/recharge methods are usually limited to handling relatively small sources of runoff such as roof drains, small parking lots, and tennis courts. Two specific infiltration methods are dry wells and infiltration trenches.

Supporting documentation shall be provided that demonstrates the permeability and infiltration capacity of all of the soils types or materials to be used in or below the infiltration/recharge area.

Dry Wells

Dry wells vary in depth from six feet to several hundred feet, depending on the depth of the permeable soil strata and the depth to bedrock. Diameters range from less than one foot up to several feet. Dry wells should be filled with crushed stone or washed two-inch gravel. Dry wells are most applicable for storing runoff from rooftops and other areas relatively free of sediment and debris.

48 Infiltration Trenches

Infiltration trenches consist of a shallow excavated trench, generally 3 to 10 feet deep, backfilled with a coarse stone aggregate, allowing for the temporary storage of storm runoff in the voids between the aggregate materials. The stored runoff then gradually infiltrates into the surrounding soil.

The permeability or final infiltration rate of the various soil classifications will be a limiting factor in the selection and location of infiltration trenches. Soil classes with infiltration rates from 0.52 inches/hour to not more than 2.4 inches/hour can be considered for the use of infiltration trenches.

The minimum recommended trench depth is 3 ft., which would include 2 ft. of the

aggregate reservoir covered by 1 ft. of soil. In general, the trench should be designed as deep as possible to minimize the surface area. However, the soil infiltration rate below the trench will dictate the maximum depth.

The trench bottom must be deeper than 2 feet to avoid freezing, and shall be kept at least 5 feet above the bedrock and/or the seasonally high ground water table in the area.

Infiltration trenches shall also be located at least 100 feet horizontally away from any water supply well.

The storage volume of the trench design is equal to the upstream runoff volume contributing to the trench times the pore volume ratio of the stone fill. The area of the bottom of the trench can be calculated using the following equation:

A

V

E

T

min





Where: Amin = area of the bottomof the trench (ft2)

V = volume of fluid storage (ft3) (trench volume less volume of stone)

E = exfiltration rate (ft/hr) (soil infiltration rate at the trench bottom) T = drain time (hr)

The volume of water exfiltrating during the filling period of the trench may be significant for permeable soils.

The rock fill of the infiltration trench shall be clean, well graded, uniform size crushed rock. Poorly graded rock has less void space available for runoff storage and shall not be

accepted.

The sides and top of the trench must be lined with geotextile to restrict the amount of sediment entering the structure. The top layer of the geotextile should be covered by a 6 to

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12 inch layer of smaller sized gravel (0.75-in. diameter). This top layer of gravel and geotextile must be replaceable. The bottom of the trench must NOT be covered with geotextile, which can become clogged with sediment, preventing infiltration.

To promote continued infiltration, the bottom of the trench should be covered with an 8 inch layer of clean sand. An observation well, consisting of a perforated 4 inch diameter vertical pipe should be installed in the trench to monitor performance. The original depth of the observation well must be marked on the top of the well.

Sediment clogging and sealing off the permeable soil is the most common cause of infiltration trench failure. Runoff from the construction site shall not be allowed to flow to the trench until construction is complete and upslope areas have been stabilized. The infiltration trench design shall include a system for removing sediment from stormwater before it enters the structure, however this system shall not be used to control sediment during construction.

Vegetative buffers of at least twenty (20) feet in width are required upstream of the trench when the contributing runoff may be carrying sediment.

Rooftop Storage

Rooftop storage is surface storage provided on flat rooftops designed for temporary ponding with special roof drain controlled release features. Rooftop storage utilizes the built-in structural capacity of rooftops to store rainfall.

Existing structures conforming to local building codes should meet the support requirements for specified snow and live loading. This allowance can be utilized for stormwater without additional support, depending upon the structural condition of the building and roof. Modifications of roof drains to allow them to function as controlled release devices would be required.

Rooftop storage could be incorporated into the design of new buildings. Directing the water to lawn or infiltration/recharge areas is recommended.

The Summit County Engineer shall not be responsible for the review or approval of the proposed structural integrity of any building. The Summit County Engineer will only be responsible for the review of the stormwater criteria portions of any rooftop storage project. Maintenance responsibility will be the responsibility of the property owner.

Limitations of rooftop storage

The main disadvantages of rooftop storage are the periodic inspection and maintenance of the facilities. Such installations may not be readily accessible. Clogging and/or

unauthorized removal of the flow control devices may occur, making routine inspections necessary.

50 Maximum Water Depth

The maximum water depth shall be determined by the developer's Design Engineer in accordance with the type of roof proposed for the building.

Live Load

The roof structure shall be designed for a minimum live load as specified in the local building codes in effect at the time of the design.

Slope

A minimum roof pitch of 0.25 in/ft to the outlet device shall be provided to assure complete drainage.

Overflow

Overflow drains shall be provided to accommodate major storms and shall be located above the maximum water depth. Roof scuppers are to be provided in parapet walls. Waterproofing

The building structure shall be designed to provide a watertight roof. Flow Control Device

The flow control device shall be in compliance with the local building code and the National Plumbing Code. Drain pipes and downspouts may be of standard design.

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6.0 PAVEMENT DRAINAGE

6.1 Introduction

Proper drainage of roadway pavement is essential to the service life of streets and to traffic safety. A good drainage design can provide lower street maintenance costs and protect pavement and subgrades from unnecessary deterioration. Water on roadways slows traffic and contributes to accidents from hydroplaning, reduced visibility and icy conditions. Effective removal of runoff is influenced by geometric characteristics such as longitudinal slope, cross slope and type of curb and gutter section. These geometric features affect the location and spacing of inlets.

Design criteria can be found in the Performance Manual.